Advancing and retracting mechanism

ABSTRACT

A power system for folding and unfolding bleachers and chair platforms utilizes a pentagon-shaped drum-sprocket for advancing and retracting chain wraps, each wrap consisting of five links of the same length and width but with the wraps varying progressively in width and length whereby the wraps are wound in close-nested relation about the drum-sprocket and are advanced from the drum-sprocket by rotation thereof through the interaction of a sprocket bar on the drum-sprocket and wedgeshaped lugs carried by the links. By employing driving force on the outer extremities of the links, a squaring effect is achieved which translates into straight-line tracking of the chain and sections.

United States Patent [i Hartman ADVANCING AND RETRACTING MECHANISM Arlin P. Hartman, Champaign, Ill.

American Seating Company, Grand Rapids, Mich.

Filed: May 4, 1971 Appl. No.: 140,115

Inventor:

Assignee:

References Cited UNITED STATES PATENTS 4/1965 McClelland 4/ 1 91 1 Mohrstadt 46/ l 09 1 ,lune 12, 1973 Primary Examiner-Othell M. Simpson Assistant Examiner-Robert C. Watson Attorney-Dawson, Tilton, Fallow & Lungmus [57] ABSTRACT A power system for folding and unfolding bleachers and chair platforms utilizes a pentagon-shaped drumsprocket for advancing and retracting chain wraps, each wrap consisting of five links of the same length and width but with the wraps varying progressively in width and length whereby the wraps are wound in close-nested relation about the drum-sprocket and are advanced from the drum-sprocket by rotation thereof through the interaction of a sprocket bar on the drumsprocket and wedge-shaped lugs carried by the links. By employing driving force on the outer extremities of the links, a squaring effect is achieved which translates into straight-line tracking of the chain and sections.

18 Claims, 16 Drawing Figures Patented June 12, 1973 3,738,612

6 Sheets-Sheet 2 INVENTOR Arlin P. Hartman BY (Du/a010, Jifimv, 7&[5910 5 a ATTORNEYS Patented June 12, 1973 6 Sheets-Sheet 3 INVENTOR Arlin P. Hartman ATTORNEYS Patented June 12, 1973 6 Sheets-Shoat 4 INVENTOR Arlin P. Hartman BY mall/d010, Jxltmzz Fallbw M66 WM ATTORNEYS Patented June 12, 1973 3,738,612

6 Sheets-Shaet 5 n" II II" 45 INENTOR f N Arhn P. Hartman y um'rv, 7 6 z ATTORNEYS Patented June 12, 1973 3,738,612

6 Sheets-Sheet 6 INVENTOR Arlln P. Hartman BY Qwm 70,240?!) ATTORNEYS v 1 ADVANCING AND RETRACTING MECHANISM BACKGROUND AND SUMMARY to be shipped as a package which can be readily installed in the field. A further problem arises in the proper nesting of the link wraps so that a reliable wrapping and unwrapping operation can be effected.

I have discovered that it is possible to provide a drum-sprocket structure, preferably in the shape of a lop-sided pentagon, which permits each of the five links of a given wrap to be of the same length so that the links of each wrap rotate concentrically about the shaft center line, and in which wedge-shaped link lugs are provided for co-operating with sprocket bars on the drum-sprocket for driving the link wraps forwardly while at the same time causing the links to nest properly when the drum-sprocket is rotated in an opposite direction The drum-sprocket is so structured as to per mit spiral wrapping of the tapered chain on the drumsprocket, and, by utilizing links which get wider and shorter adjacent to the drum-sprocket as the chain of wraps is advanced, greater rigidity is provided and straight-line tracking is brought about. That is to say, in order to permit the spiral wrappingof the chain, the drum is a lopsided pentagon in the sense that one leg of the pentagon is slightly tilted from a true pentagon side, and each successive side is also slightly tilted. This facilitates the spiral wrapping of the chain, and it also permits the axis of rotation of the hinges on superposed link pairs to lie along a common radial line from the axis of the drum. I prefer to employ a wheel race located concentrically aboutthe shafts center line for contact with wheels at the end of each link so that as the drum-sprocket is rotated and the chain is extended, the wheel race holds the links against the drumsprocket so as to convert rotary motion and thrust into linear motion and thrust. Nesting of the links is effected not only by the action of the wedge-shaped lugs, but also by the inclined sprocket bars and the cone-shaped outer ends of the wheels and the hinges on the links. While the drum-sprocket may be of different shapes, I prefer the pentagon shape because it reduces the number of links required, maintains minimum wrap diameter relative to extension, and keeps thrust surge to a minimum. Nesting of the chain occurs because it is tapered and spirally wrapped on the drum, and the length of the links in each wrap varies inversely to the width with respect to adjacent wraps with angle stops provided in the starting links at the junction of change. Floor wear is reduced because the wheels in each wrap track in a straight line on the floor and do not follow in the path of the wheels in adjacent wraps.

Because the driving forces meat or above the hinge center line which is at the top of the chain, the links stay on the floor with less tendency to buckle up off the floor under compression. A push-pull bar not only provides straight-line tracking adjustment, but also provides for fine adjustment of the chain length by providing adjusting bolts in the slotted ends.

When the carriage is provided with wheels and attached to the rear row, with the front link of the chain attached to the floor, the machine will forward fold a bleacher or folding platform by pushing and pulling it self to an extended or retracted position. The carriage provided with wheels also facilitates transporting the unit from the unloading point to the place of installation.

DRAWINGS In the accompanying drawings:

FIG. 1 is a perspective view of a bleacher or folding platform structure with the sections extended, and a portion is cut away to show the chain and empty drumsprocket or spool;

FIG. 2 is a perspective view of the unit folded in the stored position, a portion being cut away to show the loaded drum-sprocket;

FIG. 3 is a side view in elevation of the extended bleacher unit;

FIG. 4 is a view similar to FIG. 3, but showing the bleacher unit partially folded;

FIG. 5 is a view similar to FIG. 4, but showing the bleacher unit completely folded;

FIG. 6 is a broken side view of the partially wound drum-sprocket and its supporting structure, the driving sprocket being partly broken away and the motor omitted;

FIG. 7 is a horizontal view taken just above the bases or carriages of the bleacher unit and showing the extended chain and empty drum-sprocket;

FIG. 8 is a broken top plan view of the outermost wrap and a part of the adjacent wrap of links, and showing also the connection to the push and pull bar;

' FIG. 9 is a top plan view of the first wrap which is adjacent to the drum-sprocket and a. portion of the adjacent wrap;

FIG. 10 is a perspective view of the empty spool or drum-sprocket together with the wheel race;

FIG. 11 is a sectional view of the empty drumsprocket;

FIG. 12 is a view similar to FIG. 11, but showing a nearly loaded drum-sprocket;

FIG. 13 is a perspective view of the first and second links of the outermost (11th) wrap of the chain, and showing the small angle bracket opposite the wheel end;

FIG. 14 is a perspective view of the first link of the first wrap;

FIG. 15 is a sectional view, the section being taken as indicated at line 15-15 of FIG. 12, to show how all of the wedge-shaped link lugs keep the links of each wrap accurately aligned; and

FIG. 16 is a sectional view taken as indicated at line 16-16 of FIG. 12 to show how the cone-shaped wheels are nested down between the sides of the frames.

DETAILED DESCRIPTION The invention may be described generally as follows:

A drum-sprocket is provided by connecting end plates with cross members and extending spaced sprocket teeth or bars between the plates, the sprocket bars being generally V-shaped or arch-shaped with sides inclined upwardly toward the center of the arch. Upon the inclined sides of the sprocket bars are wound link wraps which are wider at the bottom and narrower at the top of the arch, the links being of the same length and width for each wrap about the drum-sprocket and with each wrap becoming progressively narrower and longer, in a direction away from the drum-sprocket. By employing upwardly extending lugs on each of the links, which, in the winding of the chain come to rest against the sprocket bar, rotation of the drum-sprocket in a forward direction causes the sprocket bars to thrust the links forwardly and the chain is advanced in a straight line tracking movement. The lugs are wedgeshaped so that when the chain is wound upon the drumsprocket in the retracting operation, the links wrap evenly without overlapping. The links are provided at their forward edges with wheels which have cone sides, and, in the wrapping of the chain on the drumsprocket, the cone ends insure against overlapping.

The links butt together at the ends and are connected by hinges at the top of the link so that the thrust will be downward as the chain is extended and the hinges permit folding of the chain only in an upward direction. By providing the foremost link with a push-pull bar, it is possible to operate the apparatus manually using the drum-sprocket as a reel. I provide also a motor for the positive operation of the drum-sprocket in the advancing and retracting operation.

While the apparatus may be employed without the use of wheels on the links, I prefer to employ the wheels on the leading edge of each link, except the foremost link, and I provide a wheel race located concentrically about the shaft center line of the drum-sprocket so that the wheels come in contact with the wheel race as the drum-sprocket is rotated. Thus, the wheels in conjunction with the wheel race hold the links against the drum-sprocket bar so that the sprocket bar remains in contact with the link lugs as the chain is extended. The force thus applied is tangent to the wrap with the line of force being roughly parallel to the floor.

Referring more specifically to the drawings, FIG. 1 shows the 30 bases designated 21 of the 30-row bleacher unit fully extended. The lower kickboard 22 is at the left or front, and the wall 23 is at the right or rear. The drum-sprocket 24 is just ahead of the wall 23, and is centered between the bleacher bases 21R and 21L (right and left as the occupants sit on the bleachers). The ll-wrap chain 25 (FIG. 7) extends from the drum-sprocket 24 to the push-pull bar 26 placed between the bases 27R and 27L of the first bleacher section.

The chain is divided into 11 wraps and each wrap has five links. The links 28 of the first wrap adjacent to the drum-sprocket as the chain is advanced are very wide from side-to-side and narrow from front to rear (FIGS. 7, 9, and 14), while the links 29 in the foremost (11th) wrap are very narrow from side-to-side and long from front to rear (FIGS. 7,8, and 13). All of the links in the same wrap are the same in size and shape except that a small angle bracket or stop 30 is secured at the leading edge 31 of the first or starting link 32 of each wrap as the chain is being wound onto the drum-sprocket for engaging the sides 36 of the trailing link of the underlying wrap to prevent reverse breaking of the chain in the open or unwrapped state.

In the specific pentagon structure illustrated best in FIG. 12, five links make one complete revolution on the drum-sprocket thus forming one wrap. In FIG. 12, ten wraps are shown' on the drum-sprocket and the 11th wrap is still to be wound up. FIG. 12 shows why the links of each successive wrap must be a little longer front to rear than the links of the wrap preceding, so

that there is a substantial difference in the length of links 28 of 1W and of the links 33 of 10W.

FIGS. 15 and 16 show that the links 26 of the first wrap (1W) are wider from side to side than the wraps of each succeeding wrap, and that the links 29 of wrap 11 (11W) are narrower from side-to-side.

As shown best in FIGS. 12 and 13, link lugs 34 are secured to the sides 36 of each link. Each of the lugs 34 includes an upright portion 34a welded to the outer surface of the corresponding side 36, and a wedgeshaped portion 34b which extends outwardly therefrom and forms an inclined guide surface 340 (see FIGS. 14 and 15). The wedge-shaped portions 34a have an important function in conjunction with the hinges in keeping the links of all of the wraps arranged side-byside on the sprocket bar. That is, as the drum of FIG. 12 is driven counterclockwise and adjacent links are rotated slightly for wrapping, the upper, narrow lead edge of the lugs 34 enters a corresponding underlying link first, and the guide surfaces 340 then engage the link sides 36 of the corresponding underlying link to center itself into nesting relation with it. The links come to rest against the sprocket bar 35, and then act as wedge spacers between the sides 36 of the links. Further, during expansion, as the drum of FIG. 12 is turned clockwise, the unwinding links of the chain are driven by action of the sprocket bar 35 pushing the lugs 34.

Each of the links, except the foremost link, is provided at its leading edge with wheels 37 which have cone-shaped sides so that they will serve as guides between the sides 36 of the links and provide bearing surface on the wheel race. The links 28 of the first wrap have their front ends secured to one of the link cross support bars 28a.

It will be noted that the wide links have two hinges 40, while the narrow links are attached by a single hinge 41, the hinges being placed at the top of the links. The hinges permit the links to wrap around the drum, but the chain cannot be bent in the reverse direction because the adjacent edges of the link sides 36, designated 39 in FIG. 13, abut, as illustrated. Further, the leading edge (during unwrapping) of the first link of a wider wrap engages the bracket 30, and this prevents reverse bending of the chain at each junction between wraps.

The push-pull bar 26 is attached to the bleacher bases 27R and L by slotted holes 42 (43) so that adjustments can be made.

As shown best in FIGS. 10, 11, and 12, the drumsprocket 24 is rotatably mounted on shaft 45 in carriage member 44, the shaft being driven by gear 46 through the action of a gear-equipped motor (not shown). A track member 47 is mounted upon carriage 44 and provides a wheel race 48. The wheel race is arranged so that the link wheels 37 come in contact with it as the drum-sprocket 24 is rotated. Thus the wheels, in conjunction with the wheel race 48, hold the links against the drum-sprocket bar so that the sprocket bar remains in contact with the link lugs 34 as the chain is extended and transferred to the floor.

It will be understood that the drum-sprocket 24 may be of any suitable construction. In the illustration given, the drum-sprocket has end plates connected by wrap adapter bars 38 and sprocket bars 35 extended therebetween. In effect, the sprocket bar 35 provides a common sprocket tooth for all of the links, and its inclined sides cooperate with the wedge lugs of the links in stacking the links evenly side-by-side as the chain is wound upon the drum-sprocket.

The advancing and retracting mechanism has been illustrated for the moving of a single bleacher section; however, it maybe used to simultaneously move two or more sections. There is a constant functional relationship between link wraps, links, sprocket bars, link. lugs, and wheel race which permits, if desired, an almost infinite increase in the number of wraps and chain length.

While I have shown the chain tapered, with the broad links attached to the drum sprocket, the chain may, if desired, be inverted, with the narrow and short links attached to the drum upside down, with successive wraps winding and nesting on top of the wraps previously wound. I prefer, however, the tapered chain arrangement shown in the drawings,- and by the term tapered I intend to include a step or pyramid chain.

While in the foregoing-specification I have shown a specific structure in considerable detail for the purpose of illustrating an embodiment of the invention, it will be understood that such details may be varied widely by those skilled in the art without departing from the spirit of my invention.

I claim:

1. Advancing and retracting mechanism comprising a frame, a drum-sprocket rotatably mounted in said frame and comprising end plates connected by cross bars and generally V-shaped sprocket bar members extending in spaced relation between said end plates, a tapered chain having one end anchored to said drumsprocket and comprising spaced segmental straps pivotally connected at their ends to form links, a plurality of said links forming a wrap, said chain comprising a plurality of said wraps of said links with said wraps be coming progressively wider toward the drum with the chain in extended position, the links of each wrap being abutted end-to-end for bending only in one direction from a straight line, said links being provided with upwardly extending link lugs engageable with said sprocket bar for advancing the chain when said drumsprocket is rotated in one direction; and means for preventing reverse bending of said chain at the junction between adjacent wraps.

2. The structure of claim 1 in which said wraps increase in length as they extend from said drumsprocket and said wraps are spirally wound on said drum-sprocket.

3. The structure of claim 1 in which said lugs are wedge-shaped in a direction transverse of the direction of travel and wherein said links are nested when wrapped on said drum-sprocket, said wedge-shaped lugs entering a previously wrapped link to guide the link being wrapped when said drum-sprocket is rotated in the other direction.

4. The structure of claim 1 in which said links straddle said v -sprocket bars as they are wound thereon.

5. The structure of claim 1 in which said links are provided at their leading edges with wheels.

6. The structure of claim 5 in which said wheels are provided with cone sides for guiding them into side-byside relation and also provide bearing surface on the wheel race as well as a flat face for bearing on the floor.

7. The structure of claim 5 in which said frame provides an arcuate wheel race for supporting said links about said drum-sprocket as the links are being advanced and retracted.

8. The structure of claim 1 in which the outermost link is anchored to a push-pull bar.

9. The structure of claim l in which said drumsprocket is provided with five equally spaced V-shaped sprocket bars and each chain wrap comprises five links and being spirally wound on said drum-sprocket.

10. The structure of claim 1 in which each link is provided with hinges at its top, front and rear edges.

11. For use with a telescoping platform structure, advancing and retracting mechanism comprising a frame, a drum-sprocket rotatably mounted in said frame, said drum-sprocket being provided with five generally V- shaped sprocket'bar members and five cross bars, a chain comprising segmental straps hingedly connected at their ends forming links butted end-to-end for bending from a straight line in one direction only and adapted to be wrapped spirally about said sprocket bars,said chain comprising a plurality of wraps of said links with each wrap having five links and said links narrowing and lengthening as the chain extends from said drum-sprocket, each of said links having upwardly extending lugs adapted to engage said sprocket bars so as to be driven thereby to advance the chain and said platform as the drum-sprocket is rotated in one direction.

12. The structure of claim 11 in which said lugs are wedge-shaped in a transverse direction and extend about midway of each link for guiding said lugs into nested engagement when said drum is rotated in an opposite direction.

13. The structure of claim 11 in which said link straps have their edges engaging said sprocket bars in closely wound relation.

14. The structure of claim 13 in which said wedgeshaped lugs guide said edges in the retracting winding operation.

15. The structure of claim 11 in which said links are provided with wheels having cone sides for guiding them in closely-assembled relation.

16. In an advancing and retracting mechanism, a drum-sprocket equipped with a plurality of archshaped sprocket bars, a chain having one link end anchored to said drum-sprocket and having progressively narrowing link wraps adapted to straddle such archshaped sprocket member as they are wound upon said drumsprocket, each of said links being provided with a vertical lug engageable with said sprocket bar whereby each sprocket bar serves as a common sprocket tooth for the links.

17. Mechanism for advancing and retracting a movable object along a surface comprising: a drumsprocket mounted for rotation about its axis; drive means for rotating said drum-sprocket in a first direction for advancing said object and a second direction for retracting said object along said surface; a plurality of rigid links spirally wound on said drum-sprocket in a plurality of superposed wraps; hinge means for pivotally connecting the forward portion of one link with the rear portion of a forward link whereby adjacent links may be rotated between an open position and a wrapped position; means associated with each link for engaging the link next forward to prevent rotation beyond said open position; and sprocket means on said drum-sprocket for engaging and pushing each link wrapped thereon as it unwinds from said drum and for feeding said unwrapped links adjacent said surface, each link of a superposed wrap having a different width than the corresponding link on the underlying wrap and nesting therewith, the forwardmost link being attached to said object whereby adjacent links assume said open position when said drum-sprocket is driven in said first direction and are prevented from bending in said hinge direction by engagement with saidsurface when unwrapped from said drum.

l8. Mechanism for advancing and retracting a movable object comprising: a frame; a drum-sprocket rotatably mounted to said frame for rotation about its axis; drive means for rotating said drum-sprocket in a first direction for advancing said object and a second direction for retracting said object; a plurality of rigid links spirally wound on said drum-sprocket in a plurality of superposed wraps; each wrap having the same number of links, and each link nesting with a corresponding link on the wrap above and below it, all of said links of a given wrap having the same length and width, each of said links of one wrap having a greater length and smaller width than the corresponding link of the wrap on which it overlies; hinge means for pivotally connecting adjacent links for rotation between a relatively closed wrapped position and a straight, open position; all of the links of a given wrap abutting adjacent such links end-to-end in said open position to restrict further relative rotation; stop means for limiting the rotation of adjacent links on different wraps beyond said open position; sprocket means extending axially of said drum and rotatable therewith; lug means on each of said links extending inwardly of said drum beyond a corresponding underlying link to engage said sprocket means and further including guide means engaging a corresponding underlying link for guiding a link onto said underlying link during wrapping; wheel means on each of said links for engaging a floor when unwrapped from said drum sprocket; and wheel race means partially surrounding said drum-sprocket for guiding each link as it is wrapped onto said drum-sprocket and unwrapped therefrom. 

1. Advancing and retracting mechanism comprising a frame, a drum-sprocket rotatably mounted iN said frame and comprising end plates connected by cross bars and generally V-shaped sprocket bar members extending in spaced relation between said end plates, a tapered chain having one end anchored to said drum-sprocket and comprising spaced segmental straps pivotally connected at their ends to form links, a plurality of said links forming a wrap, said chain comprising a plurality of said wraps of said links with said wraps becoming progressively wider toward the drum with the chain in extended position, the links of each wrap being abutted end-to-end for bending only in one direction from a straight line, said links being provided with upwardly extending link lugs engageable with said sprocket bar for advancing the chain when said drum-sprocket is rotated in one direction; and means for preventing reverse bending of said chain at the junction between adjacent wraps.
 2. The structure of claim 1 in which said wraps increase in length as they extend from said drum-sprocket and said wraps are spirally wound on said drum-sprocket.
 3. The structure of claim 1 in which said lugs are wedge-shaped in a direction transverse of the direction of travel and wherein said links are nested when wrapped on said drum-sprocket, said wedge-shaped lugs entering a previously wrapped link to guide the link being wrapped when said drum-sprocket is rotated in the other direction.
 4. The structure of claim 1 in which said links straddle said V-sprocket bars as they are wound thereon.
 5. The structure of claim 1 in which said links are provided at their leading edges with wheels.
 6. The structure of claim 5 in which said wheels are provided with cone sides for guiding them into side-by-side relation and also provide bearing surface on the wheel race as well as a flat face for bearing on the floor.
 7. The structure of claim 5 in which said frame provides an arcuate wheel race for supporting said links about said drum-sprocket as the links are being advanced and retracted.
 8. The structure of claim 1 in which the outermost link is anchored to a push-pull bar.
 9. The structure of claim 1 in which said drum-sprocket is provided with five equally spaced V-shaped sprocket bars and each chain wrap comprises five links and being spirally wound on said drum-sprocket.
 10. The structure of claim 1 in which each link is provided with hinges at its top, front and rear edges.
 11. For use with a telescoping platform structure, advancing and retracting mechanism comprising a frame, a drum-sprocket rotatably mounted in said frame, said drum-sprocket being provided with five generally V-shaped sprocket bar members and five cross bars, a chain comprising segmental straps hingedly connected at their ends forming links butted end-to-end for bending from a straight line in one direction only and adapted to be wrapped spirally about said sprocket bars, said chain comprising a plurality of wraps of said links with each wrap having five links and said links narrowing and lengthening as the chain extends from said drum-sprocket, each of said links having upwardly extending lugs adapted to engage said sprocket bars so as to be driven thereby to advance the chain and said platform as the drum-sprocket is rotated in one direction.
 12. The structure of claim 11 in which said lugs are wedge-shaped in a transverse direction and extend about midway of each link for guiding said lugs into nested engagement when said drum is rotated in an opposite direction.
 13. The structure of claim 11 in which said link straps have their edges engaging said sprocket bars in closely wound relation.
 14. The structure of claim 13 in which said wedge-shaped lugs guide said edges in the retracting winding operation.
 15. The structure of claim 11 in which said links are provided with wheels having cone sides for guiding them in closely-assembled relation.
 16. In an advancing and retracting mechanism, a drum-sprocket equipped with a plurality of arch-shaped sprocket bars, a chain having one liNk end anchored to said drum-sprocket and having progressively narrowing link wraps adapted to straddle such arch-shaped sprocket member as they are wound upon said drum-sprocket, each of said links being provided with a vertical lug engageable with said sprocket bar whereby each sprocket bar serves as a common sprocket tooth for the links.
 17. Mechanism for advancing and retracting a movable object along a surface comprising: a drum-sprocket mounted for rotation about its axis; drive means for rotating said drum-sprocket in a first direction for advancing said object and a second direction for retracting said object along said surface; a plurality of rigid links spirally wound on said drum-sprocket in a plurality of superposed wraps; hinge means for pivotally connecting the forward portion of one link with the rear portion of a forward link whereby adjacent links may be rotated between an open position and a wrapped position; means associated with each link for engaging the link next forward to prevent rotation beyond said open position; and sprocket means on said drum-sprocket for engaging and pushing each link wrapped thereon as it unwinds from said drum and for feeding said unwrapped links adjacent said surface, each link of a superposed wrap having a different width than the corresponding link on the underlying wrap and nesting therewith, the forwardmost link being attached to said object whereby adjacent links assume said open position when said drum-sprocket is driven in said first direction and are prevented from bending in said hinge direction by engagement with said surface when unwrapped from said drum.
 18. Mechanism for advancing and retracting a movable object comprising: a frame; a drum-sprocket rotatably mounted to said frame for rotation about its axis; drive means for rotating said drum-sprocket in a first direction for advancing said object and a second direction for retracting said object; a plurality of rigid links spirally wound on said drum-sprocket in a plurality of superposed wraps; each wrap having the same number of links, and each link nesting with a corresponding link on the wrap above and below it, all of said links of a given wrap having the same length and width, each of said links of one wrap having a greater length and smaller width than the corresponding link of the wrap on which it overlies; hinge means for pivotally connecting adjacent links for rotation between a relatively closed wrapped position and a straight, open position; all of the links of a given wrap abutting adjacent such links end-to-end in said open position to restrict further relative rotation; stop means for limiting the rotation of adjacent links on different wraps beyond said open position; sprocket means extending axially of said drum and rotatable therewith; lug means on each of said links extending inwardly of said drum beyond a corresponding underlying link to engage said sprocket means and further including guide means engaging a corresponding underlying link for guiding a link onto said underlying link during wrapping; wheel means on each of said links for engaging a floor when unwrapped from said drum sprocket; and wheel race means partially surrounding said drum-sprocket for guiding each link as it is wrapped onto said drum-sprocket and unwrapped therefrom. 